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Biological control of Bactericera cockerelli using a combination of parasitoids and pathogens has potential. However, their combined use could result in complex multitrophic interactions, the outcomes of which are uncertain. In this study we evaluated the effects of applications of two Beauveria bassiana isolates (pathogen) on the parasitoid Tamarixia triozae during its development on the host, B. cockerelli . Three concentrations (LC 20 , LC 50 and LC 90 values against B. cockerelli ) of the fungal isolates BB40 and GHA were applied to 4th instar B. cockerelli nymphs that had been successfully parasitized by T. triozae 3, 5, 8 or 12 days earlier at 25 °C. The greatest infection of T. triozae was observed when the calculated LC 90 was inoculated, regardless of isolate, and when the parasitoids were 3–5 days old. The lowest infection of T. triozae was observed when they were 8 and 12 days old regardless of isolate or conidial concentration. The highest parasitoid emergence was achieved in the untreated control compared with all other treatments and combinations tested. Within treatments, the highest parasitoid emergence was obtained at the lowest B. bassiana concentration tested and when the pathogen was applied to B. cockerelli supporting 12 day-old parasitoids. Longevity of adult parasitoids that had emerged from B. cockerelli nymphs was greatest in the control treatment, followed by those from nymphs treated with GHA when applied to B. cockerelli supporting 12 day-old parasitoids. In conclusion, biological control of B. cockerelli may require the synchronized use of high B. bassiana concentrations, applied only on T. triozae late pupal stage (8–12 days of development), in order to allow the parasitoid to survive and also achieve good overall pest control. [ABSTRACT FROM AUTHOR]